Retinoids, derivatives of vitamin A, have been repeatedly shown to arrest cell growth of neuroblastoma cells in vitro by causing differentiation. Clinical trials in relapsed neuroblastoma patients with bulky tumors failed to show significant responses to retinoid therapy. Subsequently, however, a sentinel randomized clinical trial demonstrated that isotretinoin(13-cis-retinoic acid), when given to patients with minimal residual disease following consolidation chemotherapy, independently improved the overall survival of patients with high-risk neuroblastoma. The treatment regimen included isotretinoin for 2 weeks followed by a 2 week rest period for 6 treatment cycles. The treatment was very well tolerated with minimal side effects. The duration of treatment, 6 months, was arbitrarily chosen and currently many institutions implement prolonged retinoic acid treatment in patients with relapsed high-risk disease, yet no formal study has been done to statistically show improved survival with prolonged biotherapy.